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G8MNY  > TECH     14.05.22 09:32z 97 Lines 3601 Bytes #29 (0) @ WW
BID : 21626_GB7CIP
Subj: 3 Band VHF Indoor Aerial
Sent: 220514/0928Z @:GB7CIP.#32.GBR.EURO #:21626 [Caterham Surrey GBR]

G0FTD's experimental INDiX antenna (c)                (corrected May 06)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
Indoor DX antenna. Meant to be hung indoors within a window frame for those of
you who can't get an outdoor antenna or operating from a hotel, bed & breakfast
or hung from a tree! The antenna is really a bi-square modified with 2 "rungs"
& lengths modelled using NEC software for optimum all round performance. You
have to make up a matching network for it which is easy enough. The antenna
can be made of wire with dowel rods to support the horizontal sections.

쳐컴컴컴컴컴컴컴10cm inwards ladder rung element
쳐컴컴컴O컴컴컴캑1.846m           O=feedpoint
쳐컴컴컴컴컴컴컴10cm inwards ladder rung element

Gain:                            Impedance:           Polarisation:
50  MHz = 2.35 dBd               18-j870              Horizontal
70  MHz = 1.14 dBd               30-j633              Horizontal
145 MHz = 4.36 dBd               185+j102             Horizontal

Radiation pattern is just like a half wave dipole figure of eight pattern but
with slight gain. There is very little high angle radiation.

The antenna is a loop 1.846 x 1.098 metres with 2 "rungs" connected 10cm
inwards which improves the gain on 145MHz by approx 1 dB. An L-Match is
inserted at the feedpoint `O`. At 50/70MHz a step down l-match is used & at
145MHz a step up. All matching networks are placed AT the feedpoint, they are
*never* placed at the rig end.

50/70 MHz matching network
----/\/\/\/\/\------>To feedpoint O
 ---                L=3uH tapped for 50/70MHz
 ---                C=20pF to 150pF approx variable
-------------------->To feedpoint O

144MHz matching network
---\/\/\/\/\/------->To feedpoint O
             ---    L=0.107uH approx
             ---    C=11.4pF approx made variable
-------------------->To feedpoint O

Alternative Matching networks using only inductors.

50 MHz
Replace C with 0.118uH inductance, Replace L with a 2.667uH inductance

Replace C with 0.137uH inductance, Replace L with 1.386uH inductance

Not possible using inductors but a coaxial matching stub may be used. Simply
take a 13cm length of RG58 coax & attach to the feedpoint but leaving the far
end open. Trim the length which should be around 11.5cm long but may vary
slightly. The coax can be coiled up to fit it in a box.

All matching networks can be made in a compact box at the feedpoint & switch in
line for each band.

Note: You must adjust the matching networks in situ & the impedance of the
antenna will vary slightly with proximity to nearby objects.

You should get about 200kHz on 50/70 MHz before the SWR reaches 2.1 but on
144MHz the entire band should easily be covered.

On 50MHz/70MHz this bandwidth is quite adequate for working the DX ends of the

This article may be reproduced on a not for profit basis, personal use or
amateur radio club website or personal website. I've taken to putting this on
the end of some of my work as I often get asked if it can be reproduced in the
above media. It would be nice if anyone does reproduce it to keep me informed
so I can point others towards it. Also if anyone uses the antenna it would be
nice to know how you get on with it.

- Andy - G0FTD@GB7SXE.#38.GBR.EU

Why Don't U send an interesting bul?

73 De John, G8MNY @ GB7CIP

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